Three children and three adults killed in a knife attack in Jersey on Sunday were all of Polish descent, it has been reported.

The victims were said to come from two families. In one family the mother, a six-year-old girl, a boy of 18 months, and the mother's father died, Sky News reported, citing the Polish embassy as its source. They were said to be Polish citizens.

Another woman and a girl also died and are said to be of mixed Polish and Jersey citizenship. One of the women died from her injuries after surgery.

A 30-year-old man in police custody at Jersey general hospital, also said to be Polish, is in a stable condition after undergoing surgery for knife injuries. The suspect was related to the family of four.

Jersey police would only confirm that one family was of Polish origin, although both were local. Det Supt Stewart Gull, who is leading the investigation, told a press conference the names of those killed in the incident at a flat in St Helier were unlikely to be released until later this week.

Two victims were found outside the building and four inside. Gull said the Polish family lived at the flat where the incident took place but would only say the other family were from Jersey.

The suspect was part of the Polish family. No motive was known and it would be "pure speculation" to try to guess.

He did not know of any police involvement with the families in the past. "Police are working closely with leaders from both the local St Helier and Polish communities to support the investigation and local people affected by the tragic incident.

"Forensic analysis of the grounds around the scene is under way, and crime scene investigators from Devon and Cornwall police are en route to the island to support the complex examination of the scene and evidence."

Home Office pathologists have arrived in Jersey to conduct postmortems. Jersey's chief minister senator, Terry Le Sueur, said the island was "saddened and shocked". He said the authorities would ensure support and counselling to local people as he appealed for the public not to speculate on the incident at a flat in St Helier.

States of Jersey police said that officers were called to a flat in Victoria Crescent, Upper Midvale Road, in the town just after 3pm after reports of a multiple stabbing.

The immediate area was sealed off while a major incident room was set up at police headquarters in St Helier. The area, made up of smart houses, many of them converted into flats, was quiet on Monday morning though the scene was still being guarded by police, including armed officers. A forensic team could be seen searching through undergrowth.

A local resident said that on Sunday he saw a woman's body on the ground covered in blood and watched as paramedics carried the bodies of two little girls out of the flat.

He said: "I've never seen so much blood. They were completely limp. The paramedics were crying. They were completely drenched in blood. One of the paramedics had to change his clothes."

He said the girls were blonde and were both wearing dresses. He said the woman was lying in the road and was also covered in blood, and appeared lifeless.

The man, who would give only his first name, John, said he had run to the scene after hearing a woman scream.

"She shouted, 'Please help me! Please, God, help me!'" he said. Another witness, Andre Thorpe, said two ambulances attended the scene, which was within a mile of the ambulance station.

"Then four or five police vehicles came," he said. "They were trying to access a private house in the crescent. It was an old Victorian terrace – a lot of them are split into flats.

"I saw police come running out with a child. It was a small child. I just saw the legs. They went off in an ambulance. When the paramedic came back, her shirt was covered in blood."

He said the incident had happened in a secluded area and not on a main road, adding: "You have to drive up a dead end to get in. You would not happen to be passing."

The attacks occurred in the flat and in the street, according to police, who are not believed to be looking for anyone else but are continuing with inquiries.

A force spokeswoman said: "This is a close-knit community and we have had a fantastic response with information from locals that they believe would be useful."

Gull said on Sunday that a number of the victims had been attacked "with a knife or knives" and that it had been a "pretty traumatic" incident for emergency teams to deal with.

The island has fewer than 100,000 residents and crime has recently fallen to its lowest level in the past 10 years.

"Jersey is an incredibly safe island, probably one of the safest places in the western world, and incidents of this nature are an extremely rare occurrence," according to Gull, who later said that he believed the last murder on the island took place in 2004.

Police figures for the first half of this year show violent crime down 20%. Nine out of 10 adults in Jersey considered their neighbourhood very or fairly safe, the 2010 Jersey annual social survey found.

Gull, who led the Ipswich serial murders inquiry in 2006, said: "It goes without saying that when you are dealing with multiple deaths of men and women and, in particular, young children, you would be inhuman not to be shaken yourself."

Le Sueur said: "I was deeply saddened and shocked by yesterday's tragic events and I would like to extend my sincerest condolences to the friends and relatives of those involved.

"This is now a police inquiry and we fully support States of Jersey police officers as they carry out their investigations.

"I have every confidence in the ability and professionalism of our police force in handling this investigation. We must now avoid speculation and allow them to continue with this important work.

"Jersey is a very safe place and events of this terrible nature are very rare. This has greatly shocked the island's community. Many will need support and counselling in the days ahead and we will ensure this is provided," Le Sueur added, thanking emergency services and "especially paramedics and hospital staff for their tireless work".

Monsignor Nicholas France, head of the Catholic church in Jersey, said there was "great distress and anxiety" at the horrifying attack, and that prayers had been offered at a Polish mass on Sunday night for those involved. He told BBC Breakfast: "One's picked up a great sense of sadness that this could happen, especially to a family. On a small island like this it's a wound for the whole family, the whole community."

The hospital's emergency department was closed for more than two hours due to the volume of victims being admitted, and staff were called in on days off to help colleagues.

A number of witnesses who have come forward have been interviewed by police. They urged anyone with information to contact them on 01534 612612.